There are in existence power braking systems wherein engine vacuum is applied to a diaphragm associated with the piston of a master cylinder or the brake pedal (different types of systems). The present invention relates to a force booster that utilizes energy from external hydraulic pressure source in lieu of engine vacuum. Objects of the present invention are to provide a force booster that (1) is relatively compact, (2) that utilizes axial forces for minimizing side thrust and wear, (3) that is capable of use with conventional master cylinders, (4) that requires a minimum number of external connections to the hydraulic pressure source, and (5) that has a fail-safe operation in event of loss of hydraulic pressure.